Display device and method for selecting optical film of display device

ABSTRACT

There is provided a display device exhibiting a good color reproducibility, even when observed through polarized sunglasses. A display device comprises a polarizer a and an optical film X on a surface on a light emitting surface side of a display element, wherein L 1 , which is the light incident vertically on the optical film X, among light incident on the optical film X from the display element side, satisfies a specific condition, and L 2 , which is the light emitting vertically from the light emitting surface side of the optical film X, and passing through a polarizer b having the absorption axis parallel to the absorption axis of the polarizer a, satisfies a specific condition.

TECHNICAL FIELD

The present invention relates to a display device and a method ofselecting an optical film of the display device.

BACKGROUND ART

In display devices represented by liquid crystal display devices, theperformance, such as luminance, resolution, and a color gamut, israpidly progressing. Additionally, in proportion to the progress in theperformance, display devices assuming the outdoor use, such as personaldigital assistants and car navigation systems, are increasing.

In environments such as an outdoor environment with strong sunlight,there are cases where display devices are observed through sunglasses(hereinafter referred to as “the polarized sunglasses”) having apolarizing function in order to reduce the glare.

When a display device includes a polarization plate, there is a problem(hereinafter referred to as “the blackout”) that a screen becomes darkand hard to see when the absorption axis of polarization of the displaydevice becomes orthogonal to the absorption axis of polarization of thepolarized sunglasses.

In order to solve the above-mentioned problem, the means of PTL1 hasbeen proposed.

CITATION LIST Patent Literature

PTL1: JP 2011-107198 A

SUMMARY OF INVENTION Technical Problem

PTL1 is characterized by arranging a polymer film having a 3000 to 30000nm retardation at a specific angle on the visual recognition side of apolarization plate in a liquid crystal display device using a whitelight emitting diode (white LED) as a backlight source. The problem ofthe blackout can be solved by the means of PTL1.

Additionally, in PTL1, the interference color (rainbow unevenness)peculiar to a retardation value is prevented in the liquid crystaldisplay device using a white light emitting diode (white LED) as abacklight source.

On the other hand, recently, in order to improve luminance, resolution,a color gamut, etc., the light sources and display elements of displaydevices have been diversified. For example, as the light source of thebacklight of a liquid crystal display device, a white LED, which is usedin PTL1, is widely used. However, recently, liquid crystal displaydevices are beginning to be proposed that use a quantum dot as the lightsource of the backlight. Additionally, though the mainstream of thepresent display element is a liquid crystal display element, theutilization of an organic EL element is spreading recently.

When these recent display devices are observed through the polarizedsunglasses, even if the above-mentioned problems (the blackout and therainbow unevenness) do not occur, another problem may take place in thecolor reproducibility.

The present invention aims to provide a display device having a goodcolor reproducibility, and a method of selecting an optical sheet of thedisplay device.

Solution to Problem

In order to solve the above-mentioned problems, the inventors focused onthe difference between the liquid crystal display device using the whiteLED, which was the conventional mainstream, and the display device,which is currently under development. As a result, it was found that,compared with the liquid crystal display devices using white LEDs, therecent display devices have a sharper optical spectrum of RGB and awider color gamut (the width of a reproducible color), and that, due tothe wide color gamut, a problem tends to occur in the colorreproducibility when light passes through an optical film having aretardation and the absorption axis of polarization.

Additionally, as a result of further study by the inventors, it wasfound that, in the display devices having a wide color gamut, the longerthe wavelength becomes, the color reproducibility problem is more likelyto occur, and that mere consideration of the optical spectrum of a lightsource as in PTL1 cannot solve the problem, and it is necessary toconsider the optical spectrum of a display element, which resulted insolving the above-mentioned problems.

The present invention provides a display device and a method ofselecting an optical film of the display device as follows.

[1] A display device including a polarizer a and an optical film X on asurface on a light emitting surface side of a display element, andsatisfying the condition 1-1 and the condition 2-1 as follows:

<Condition 1-1>

Let L₁ represent the light incident vertically on the optical film X,among light incident on the optical film X from the display elementside. The intensity of the L₁ is measured every 1 nm. It is assumed thatthe blue wavelength band range from 400 nm to less than 500 nm, thegreen wavelength band range from 500 nm to less than 600 nm, and the redwavelength band range from 600 nm to 780 nm. Let B_(max) represent themaximum intensity of the blue wavelength band of the L₁, G_(max)represent the maximum intensity of the green wavelength band of the L₁,and R_(max) represent the maximum intensity of the red wavelength bandof the L₁.

Let L₁λ_(B) represent the wavelength showing the B_(max), L₁λ_(G)represent the wavelength showing the G_(max), and L₁λ_(R) represent thewavelength showing the R_(max).

Let +α_(B) represent the minimum wavelength showing a ½ or less of theintensity of the B_(max), and located in the plus direction side ofL₁λ_(B), −α_(G) represent the maximum wavelength showing a ½ or less ofthe intensity of the G_(max), and located in the minus direction side ofL₁λ_(G), +α_(G) represent the minimum wavelength showing a ½ or less ofthe intensity of the G_(max), and located in the plus direction side ofL₁λ_(G), and −α_(R) represent the maximum wavelength showing a ½ or lessof the intensity of the R_(max), and located in the minus direction sideof L₁λ_(R).

L₁λ_(B), L₁λ_(G), L₁λ_(R), +α_(B), −α_(G), +α_(G) and −α_(R) satisfy thefollowing relationships (1) to (4).+α_(B) <L ₁λ_(G)  (1)L ₁λ_(B)<−α_(G)  (2)+α_(G) <L ₁λ_(R)  (3)L ₁λ_(G)<−α_(R)  (4)<Condition 2-1>

Let L₂ represent the light that emits vertically from the light emittingsurface side of the optical film X, and passes through a polarizer bhaving the absorption axis parallel to the absorption axis of thepolarizer a. The intensity of the L₂ is measured every 1 nm. Thewavelength at which the inclination of the optical spectrum of the L₂changes from negative to positive is assumed to be a bottom wavelength,and the wavelength at which the inclination of the optical spectrum ofthe L₂ switches from positive to negative is assumed to be a peakwavelength.

Let −β_(R) represent the maximum wavelength showing a ⅓ or less of theintensity of the R_(max), and located in the minus direction side ofL₁λ_(R), and +β_(R) represent the minimum wavelength showing a ⅓ or lessof the intensity of the R_(max), and located in the plus direction sideof L₁λ_(R).

A wavelength band ranging from −β_(R) to +β_(R) in the range of 600 nmto 780 nm includes one or more bottom wavelengths and one or more peakwavelengths.

A method of selecting an optical film of a display device including apolarizer a and the optical film on a surface on a light emittingsurface side of a display element, wherein, when incident light beingincident on the optical film satisfies the above condition 1-1, theoptical film satisfying the above condition 2-1 is selected.

Advantageous Effects of Invention

The display device of the present invention can suppress the reductionof the color reproducibility when observed through the polarizedsunglasses. Additionally, the method of selecting the optical film ofthe display device of the present invention can efficiently select theoptical film that can suppress the reduction of the colorreproducibility when observed through the polarized sunglasses.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing one embodiment of a displaydevice of the present invention.

FIG. 2 is an example of an optical spectrum of light (L₁) incident on anoptical film from the display element side in a display device thatincludes a polarizer a and the optical film on a three-color independenttype organic EL display element having a micro-cavity structure.

FIG. 3 is an example of an optical spectrum of light (L₁) incident on anoptical film from the display element side in a display device in whicha display element is a liquid crystal display element, a light source ofa backlight is a cold cathode fluorescent tube (CCFL), and that includesa polarizer a and the optical film on the display element.

FIG. 4 is an example of an optical spectrum of light (L₁) incident on anoptical film from the display element side in a display device in whicha display element is a liquid crystal display element, a light source ofa backlight is a white LED, and that includes a polarizer a and theoptical film on the display element.

FIG. 5 is an example of an optical spectrum of light (L₁) incident on anoptical film from the display element side in a display device in whicha display element is a liquid crystal display element, a primary lightsource of a backlight is a blue LED, a secondary light source is aquantum dot, and that includes a polarizer a and the optical film on thedisplay element.

FIG. 6 is an example of an optical spectrum of light (L₂) that isemitted from an optical film of a display device that includes apolarizer a and the optical film on a three-color independent typeorganic EL display element having a micro-cavity structure, and passesthrough a polarizer b having the absorption axis parallel to theabsorption axis of the polarizer a.

FIG. 7 is an example of an optical spectrum of light (L₂) that isemitted from an optical film of a display device in which a displayelement is a liquid crystal display element, a light source of abacklight is a cold cathode fluorescent tube (CCFL), and that includes apolarizer a and the optical film on the liquid crystal display element,and passes through a polarizer b having the absorption axis parallel tothe absorption axis of the polarizer a.

FIG. 8 is an example of an optical spectrum of light (L₂) that isemitted from an optical film of a display device in which a displayelement is a liquid crystal display element, a light source of abacklight is a white LED, and that includes a polarizer a and theoptical film on the liquid crystal display element, and passes through apolarizer b having the absorption axis parallel to the absorption axisof the polarizer a.

FIG. 9 is an example of an optical spectrum of light (L₂) that isemitted from an optical film of a display device in which a displayelement is a liquid crystal display element, a primary light source of abacklight is a blue LED, a secondary light source is a quantum dot, andthat includes a polarizer a and the optical film on the display element,and passes through a polarizer b having the absorption axis parallel tothe absorption axis of the polarizer a.

FIG. 10 is a diagram in which the optical spectrum of FIG. 2 and theoptical spectrum of FIG. 6 are superimposed on each other.

FIG. 11 is a diagram in which the optical spectrum of FIG. 3 and theoptical spectrum of FIG. 7 are superimposed on each other.

FIG. 12 is a diagram in which the optical spectrum of FIG. 4 and theoptical spectrum of FIG. 8 are superimposed on each other.

FIG. 13 is a diagram in which the optical spectrum of FIG. 5 and theoptical spectrum of FIG. 9 are superimposed on each other.

FIG. 14 is another example of an optical spectrum of light (L₁) incidenton an optical film from the display element side in a display device inwhich a display element is a liquid crystal display element, a primarylight source of a backlight is a blue LED, a secondary light source is aquantum dot, and that includes a polarizer a and the optical film on thedisplay element.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention are described.

[Display Device]

A display device of the present invention includes a polarizer a and anoptical film X on a surface on the light emitting surface side of adisplay element, and satisfies the following conditions 1-1 and 2-1.

<Condition 1-1>

Let L₁ represent the light incident vertically on the optical film X,among light incident on the optical film X from the display elementside. The intensity of the L₁ is measured every 1 nm. It is assumed thatthe blue wavelength band range from 400 nm to less than 500 nm, thegreen wavelength band range from 500 nm to less than 600 nm, and the redwavelength band range from 600 nm to 780 nm. Let B_(max) represent themaximum intensity of the blue wavelength band of the L₁, G_(max)represent the maximum intensity of the green wavelength band of the L₁,and R_(max) represent the maximum intensity of the red wavelength bandof the L₁.

Let L₁λ_(B) represent the wavelength showing the B_(max), L₁λ_(G)represent the wavelength showing the G_(max), and L₁λ_(R) represent thewavelength showing the R_(max).

Let +α_(B) represent the minimum wavelength showing a ½ or less of theintensity of the B_(max), and located in the plus direction side ofL₁λ_(B), −α_(G) represent the maximum wavelength showing a ½ or less ofthe intensity of the G_(max), and located in the minus direction side ofL₁λ_(G), +α_(G) represent the minimum wavelength showing a ½ or less ofthe intensity of the G_(max), and located in the plus direction side ofL₁λ_(G), and −α_(R) represent the maximum wavelength showing a ½ or lessof the intensity of the R_(max), and located in the minus direction sideof L₁λ_(R).

L₁λ_(B), L₁λ_(G), L₁λ_(R), +α_(B), −α_(G), +α_(G) and −α_(R) satisfy thefollowing relationships (1) to (4).+α_(B) <L ₁λ_(G)  (1)L ₁λ_(B)<−α_(G)  (2)+α_(G) <L ₁λ_(R)  (3)L ₁λ_(G)<−α_(R)  (4)<Condition 2-1>

Let L₂ represent the light that emits vertically from the light emittingsurface side of the optical film X, and passes through a polarizer bhaving the absorption axis parallel to the absorption axis of thepolarizer a. The intensity of the L₂ is measured every 1 nm. Thewavelength at which the inclination of the optical spectrum of the L₂changes from negative to positive is assumed to be a bottom wavelength,and the wavelength at which the inclination of the optical spectrum ofthe L₂ switches from positive to negative is assumed to be a peakwavelength.

Let −β_(R) represent the maximum wavelength showing a ⅓ or less of theintensity of the R_(max), and located in the minus direction side ofL₁λ_(R), and +β_(R) represent the minimum wavelength showing a ⅓ or lessof the intensity of the R_(max), and located in the plus direction sideof L₁λ_(R).

A wavelength band ranging from −β_(R) to +β_(R) in the range of 600 nmto 780 nm includes one or more bottom wavelengths and one or more peakwavelengths.

FIG. 1 is a cross-sectional view showing an embodiment of a displaydevice of the present invention. A display device (100) of FIG. 1includes a polarizer a (40) and an optical film X (20) on a lightemitting surface of a display element (10). In the display device (100)of FIG. 1, an organic EL display element (10 a) is used as the displayelement. Additionally, the display device (100) of FIG. 1 has anotheroptical film (30) provided between the polarizer a (40) and the opticalfilm X (20).

Further, though the other optical film (30) is provided between thepolarizer a (40) and the optical film X (20) in FIG. 1, the position ofthe other optical film (30) may be between the display element and thepolarizer a, or may be closer to an observer than the optical film X.Additionally, when the display element of the display device is a liquidcrystal display element, a backlight, which is not shown, is requiredbehind the liquid crystal display element.

(Condition 1-1)

The condition 1-1 is a condition indicating that the optical spectrum ofRGB (red, green, blue) of the display device is sharp. The condition 1-1will be described in more detail, referring to the drawings.

FIG. 2 is an example of the optical spectrum at the time of measuringthe intensity of light (L₁) incident vertically on an optical film fromthe display element side every 1 nm, when a display element outputswhite light, in a display device that includes a polarizer a and theoptical film on a three-color independent type organic EL displayelement having a micro-cavity structure. Further, in the opticalspectrum of FIG. 2, the intensity of each wavelength is standardized bysetting the maximum intensity to 100.

In FIG. 2, B_(max) indicates the maximum intensity in a blue wavelengthband (from 400 nm or more to less than 500 nm), G_(max) indicates themaximum intensity in a green wavelength band (from 500 nm or more toless than 600 nm), and R_(max) indicates the maximum intensity in a redwavelength band (from 600 nm or more to 780 nm or less).

Additionally, in FIG. 2, L₁λ_(B) indicates the wavelength showingB_(max), L₁λ_(G) indicates the wavelength showing G_(max), and L₁λ_(R)indicates the wavelength showing R_(max).

In addition, in FIG. 2, +α_(B) indicates the minimum wavelength thatshows a ½ or less of the intensity of B_(max), and is located on theplus direction side of L₁λ_(B). −α_(G) indicates the maximum wavelengththat shows a ½ or less of the intensity of G_(max), and is located onthe minus direction side of L₁λ_(G). +α_(G) indicates the minimumwavelength that shows a ½ or less of the intensity of G_(max), and islocated on the plus direction side of L₁λ_(G). −α_(R) indicates themaximum wavelength that shows a ½ or less of the intensity of R_(max),and is located on the minus direction side of L₁λ_(R).

In the optical spectrum of FIG. 2, each spectrum of RGB is sharp, andL₁λ_(B), L₁λ_(G), L₁λ_(R), +α_(B), −α_(G), +α_(G) and −α_(R) satisfy thefollowing relationships (1) to (4).+α_(B) <L ₁λ_(G)  (1)L ₁λ_(B)<−α_(G)  (2)+α_(G) <L ₁λ_(R)  (3)L ₁λ_(G)<−α_(R)  (4)

FIG. 3 is an example of the optical spectrum at the time of measuringthe intensity of light (L₁) incident vertically on an optical film fromthe display element side every 1 nm, when a display element outputswhite light, in a display device in which the display element is aliquid crystal display element, a light source of a backlight is a coldcathode fluorescent tube (CCFL), and that includes a polarizer a and theoptical film on the display element. Also in FIG. 3, each opticalspectrum of RGB is sharp, and satisfies the above-mentionedrelationships (1) to (4). Further, in the optical spectrum of FIG. 3,the intensity of each wavelength is standardized by setting the maximumintensity to 100.

FIG. 4 is an example of an optical spectrum of light (L₁) incidentvertically on an optical film from the display element side, when adisplay element outputs white light, in a display device in which thedisplay element is a liquid crystal display element, a light source of abacklight is a white LED, and that includes a polarizer a and theoptical film on the display element. In FIG. 4, the optical spectrum ofB (blue) is sharp, and the optical spectrum of G (green) iscomparatively sharp. Therefore, though the above-mentioned relationships(1) to (3) are satisfied, the above-mentioned relationship (4) is notsatisfied, since the optical spectrum of R (red) is broad. Further, inthe optical spectrum of FIG. 4, the intensity of each wavelength isstandardized by setting the maximum intensity to 100.

FIG. 5 is an example of an optical spectrum at the time of measuring theintensity of light (L₁) incident vertically on an optical film from thedisplay element side every 1 nm, when a display element outputs whitelight, in a display device in which the display element is a liquidcrystal display element, a primary light source of a backlight is a blueLED, a secondary light source is a quantum dot, and that includes apolarizer a and the optical film on the display element. Also in FIG. 5,each optical spectrum of RGB is sharp, and satisfies the above-mentionedrelationships (1) to (4). Further, in the optical spectrum of FIG. 5,the intensity of each wavelength is standardized by setting the maximumintensity to 100.

Next, the relationship between the optical spectrum of RGB and the widthof color gamut is described.

The color gamut that can be reproduced by mixing three colors of RGB isshown by a triangle on a CIE-xy chromaticity diagram. The triangledefines the apex coordinates of each color of RGB, and is formed byconnecting each apex. In the CIE-xy chromaticity diagram, when eachoptical spectrum of RGB is sharp, as for the apex coordinates of R, thevalue of x becomes large and the value of y becomes small, as for theapex coordinates of G, the value of x becomes small and the value of ybecomes large, and as for the apex coordinates of B, the value of xbecomes small and the value of y becomes small. That is, when eachoptical spectrum of RGB is sharp, the area of the triangle made byconnecting the apex coordinates of each of the RGB colors in the CIE-xychromaticity diagram becomes large, and the width of a reproduciblecolor gamut becomes wide. Further, widening the width of color gamutleads to the improvement of impressiveness and sense of presence of amoving image.

As a standard for representing the color gamut, there is “ITU-RRecommendation BT. 2020 (hereinafter referred to as “BT. 2020”)” etc.ITU-R is the abbreviation for “International TelecommunicationUnion—Radiocommunication Sector”, and ITU-R Recommendation BT. 2020 isan international standard for the color gamut of Super Hi-Vision. Whenthe cover rate of BT. 2020 based on the CIE-xy chromaticity diagramrepresented by the following formula is within a range described later,it is possible to more easily improve the impressiveness and sense ofpresence of a moving image.

<The Formula Representing the Cover Rate of BT. 2020>[The overlapping area in the area of the CIE-xy chromaticity diagram ofL ₁ with the area of the CIE-xy chromaticity diagram of BT. 2020/thearea of the CIE-xy chromaticity diagram of BT. 2020]×100(%)

Next, the problem of the color reproducibility is described.

In a display device having a wide color gamut satisfying the condition1-1, when an image is observed through the polarized sunglasses, theproblem of the color reproducibility (especially, the problem of thecolor reproducibility caused by red) tends to occur. It seems that thisis because the cycle of change of the intensity of the optical spectrumof L₂ becomes large, due to the influence of the retardation value andthe wavelength dependency of the birefringence index of the opticalfilm.

FIGS. 6 to 9 are optical spectra of light (L₂) that is obtained bycausing L₁, having the optical spectra of FIGS. 2 to 5, to enter anoptical film, having a retardation value: 11,000 nm, so as to emitvertically from the light emitting surface side of the optical film, andto pass through a polarizer b having the absorption axis parallel to theabsorption axis of the polarizer a. The optical spectrum of L₂ can beconsidered as an optical spectrum that is visually recognized throughthe polarized sunglasses. Referring to the optical spectra of FIGS. 6 to9, the larger the wavelength becomes, the larger the cycle of change ofthe intensity of the optical spectrum of L₂ becomes. Further, in theoptical spectra of L₂ in FIGS. 6 to 9, the intensity of each wavelengthis standardized by setting the maximum intensity of the optical spectraof L₁ to 100. Additionally, L₂ in FIGS. 6 to 9 is the light of Ppolarization (the polarization in the vertical direction with respect tothe optical film X).

FIG. 10 is a diagram in which FIG. 2 and FIG. 6 are superimposed on eachother, FIG. 11 is a diagram in which FIG. 3 and FIG. 7 are superimposedon each other, FIG. 12 is a diagram in which FIG. 4 and FIG. 8 aresuperimposed on each other, and FIG. 13 is a diagram in which FIG. 5 andFIG. 9 are superimposed on each other.

L₁ in FIG. 12 does not satisfy the condition 1-1. In FIG. 12, much ofthe optical spectrum of L₂ is included in the optical spectrum of L₁.That is, as shown in FIG. 12, when the optical spectrum of L₁ is notsharp, the problem of the color reproducibility hardly occurs, since abig difference is hardly produced between the optical spectrum of L₁ andthe optical spectrum of L₂, though the color gamut is narrow.

On the other hand, as shown in FIGS. 10, 11 and 13, when the opticalspectrum of L₁ is sharp, the problem of the color reproducibility easilyoccurs, since the optical spectrum of L₂ is hardly included in theoptical spectrum of L₁. Especially, in the red (R) wavelength band, theoptical spectrum of L₂ is hardly included in the optical spectrum of L₁.This is because the cycle of change of the intensity of the opticalspectrum of L₂ becomes large with the increase in the wavelength, due tothe influence of the retardation value and the wavelength dependency ofthe birefringence index, etc. of the optical film.

In the present invention, it is preferable that the optical spectra ofL₁ and L₂ be the optical spectra at the time of causing the displayelement to output white light. These optical spectra can be measured byusing a spectrophotometer. At the time of measurement, the photodetectorof the spectrophotometer is placed to be perpendicular to the lightemitting surface of the display device, and the viewing angle is set to1 degree. Additionally, it is preferable that the light to be measuredbe the light passing through the center of an effective display area ofthe display device. The optical spectra can be measured by aspectroradiometer CS-2000 made by KONICA MINOLTA, INC., for example.

In addition, “the area of the CIE-xy chromaticity diagram of L₁”, whichis needed when the cover rate of BT. 2020 is calculated, can becalculated by measuring the x values and the y values of the CIE-Yxycolor system at the time of outputting red (R) light, green (G) light,and blue (B) light, respectively, and using “the apex coordinates of red(R)”, “the apex coordinates of green (G)”, and “the apex coordinates ofblue (B)”, which are obtained from the measurement results. The x valueand the y value of the CIE-Yxy color system can be measured by aspectroradiometer CS-2000 made by KONICA MINOLTA, INC., for example.

(Condition 2-1)

The condition 2-1 represents the condition for preventing the problem ofthe color reproducibility from being produced.

Further, in the condition 2-1, “the polarizer b” substantially means“the polarizer of the polarized sunglasses”. That is, in the condition2, “the light (L₂) that emits vertically from the optical film X, andpasses through a polarizer b having the absorption axis parallel to theabsorption axis of the polarizer a” means “the light that emits from theoptical film X, and passes through the polarizer of the polarizedsunglasses (the light recognized by a human through the polarizedsunglasses)”.

Additionally, in the condition 2-1, the definition as “a wavelength bandranging from −β_(R) to +β_(R) in the range of 600 nm to 780 nm” meansthat, for the wavelength band of less than 600 nm or more than 780 nm,even if this wavelength is in a range from −β_(R) to +β_(R), the bottomwavelength and the peak wavelength are not counted.

The solid line in FIG. 10 corresponds to that in FIG. 2, and the brokenline in FIG. 10 corresponds to that in FIG. 6.

In FIG. 10, −β_(R) indicates the maximum wavelength showing a ⅓ or lessof the intensity of R_(max), and is located in the minus direction sideof L₁λ_(R). Additionally, in FIG. 10, +β_(R) indicates the minimumwavelength showing a ⅓ or less of the intensity of R_(max), and islocated in the plus direction side of L₁λ_(R).

When the wavelength at which the inclination of the optical spectrum ofL₂ changes from negative to positive is assumed to be the bottomwavelength, and the wavelength at which the inclination of the opticalspectrum of L₂ switches from positive to negative is assumed to be thepeak wavelength, the broken line in FIG. 10 includes one or more bottomwavelengths and one or more peak wavelengths in a wavelength bandranging from −β_(R) to +β_(R) in the range of 600 nm to 780 nm, andsatisfies the condition 2-1.

When the condition 2-1 is satisfied, it means that one or more crests ofthe wavelength band of the red (R) of L₂ are included in the crest nearthe maximum intensity of the red (R) of L₁. That is, when the condition2-1 is satisfied, it is possible to suppress the problem of the colorreproducibility due to red (R), since there is less difference betweenthe optical spectrum of the wavelength band of the red (R) of L₁, andthe optical spectrum of the wavelength band of the red (R) of L₂.

On the other hand, when the condition 2-1 is not satisfied, it meansthat no crest of the wavelength band of the red (R) of L₂ is included inthe crest near the maximum intensity of the red (R) of L₁. For thisreason, when the condition 2-1 is not satisfied, the colorreproducibility is deteriorated due to the red (R).

In the wavelength band of the red (R) of L₂, while red (R) is high nextto green (G) with respect to human visibility, the cycle of the opticalspectrum becomes long due to the wavelength dispersion property of theretardation. For this reason, when the optical spectrum of L₁ is sharp,the condition 2-1 cannot be satisfied with a usual design, and the colorreproducibility is deteriorated due to red (R) of which human visibilityis high. The present invention enables the suppression of deteriorationof the color reproducibility in consideration of the wavelengthdispersion property of the retardation (especially, the wavelengthdispersion property of the retardation influenced by the wavelengthdependency of the birefringence index).

Further, in the liquid crystal display device using the white LED as thelight source of the backlight, which was conventionally the mainstream,the optical spectrum of the red (R) of L₁ is broad as shown in FIG. 4.Therefore, the crest of the wavelength band of the red (R) of L₂ can beeasily included in the crest near the maximum intensity of the red (R)of L₁. That is, the deterioration of the color reproducibility due tored (R) during observation through the polarized sunglasses is a problemthat cannot occur in the liquid crystal display device using the whiteLED, which was the conventional mainstream.

In the condition 2-1, L₂ is polarized, and may be P polarization or maybe S polarization. Further, many of the usual polarized sunglasses cut Spolarization. For this reason, it is preferable that the condition 2-1be satisfied when L₂ is P polarization.

In the condition 2-1, it is preferable that a wavelength band rangingfrom −β_(R) to +β_(R) in the range of 600 nm to 780 nm include two ormore above-mentioned bottom wavelengths and two or more above-mentionedpeak wavelengths.

Additionally, it is preferable for the display device of the presentinvention to satisfy one or more of the following conditions 2-2 to 2-4,in order to suppress the problem of the color reproducibility more. Bysatisfying one or more of the conditions 2-2 to 2-4, it is possible tosuppress the deterioration of the color reproducibility due to red (R)even more.

<Condition 2-2>0.40≤[the total intensity of the L ₂ in the wavelength band ranging from−β_(R) to +β_(R) in the range of 600 nm to 780 nm/the total intensity ofthe L ₁ in the wavelength band ranging from −β_(R) to +β_(R) in therange of 600 nm to 780 nm]

Regarding the condition 2-2, it is more preferable to satisfy 0.45≤theright-hand side, and it is still more preferable to satisfy 0.47≤theright-hand side.

Further, in the condition 2-2, the definition as “the wavelength bandranging from −β_(R) to +β_(R) in the range of 600 nm to 780 nm” meansthat the wavelength band of less than 600 nm or more than 780 nm is notused for obtaining the total intensity, even if this wavelength bandranges from −β_(R) to +β_(R).

In the condition 2-2 and the condition 2-4 described below, it ispreferable that the total intensity of L₁ and L₂ be calculated byassuming the angle θ between the absorption axis (the vibratingdirection of linear polarization) of the polarizer of polarizer a andthe slow axis of the optical film X to be 45 degrees.

<Condition 2-3>

Let −α_(R) represent the maximum wavelength showing a ½ or less of theintensity of the R_(max), and located in the minus direction side ofL₁λ_(R), and +α_(R) represent the minimum wavelength showing a ½ or lessof the intensity of the R_(max), and located in the plus direction sideof L₁λ_(R).

A wavelength band ranging from −α_(R) to +α_(R) in the range of 600 nmto 780 nm includes one or more above-mentioned bottom wavelengths of theL₂ and one or more peak wavelengths of the L₂.

Further, in the condition 2-3, the definition as “a wavelength bandranging from −α_(R) to +α_(R) in the range of 600 nm to 780 nm” meansthat, for the wavelength band of less than 600 nm or more than 780 nm,even if this wavelength band is in a range from −α_(R) to +α_(R), thebottom wavelength and the peak wavelength are not counted.

In the conditions 2-3, it is more preferable that a wavelength bandranging from −α_(R) to +α_(R) in the range of 600 nm to 780 nm includetwo or more above-mentioned bottom wavelengths and two or moreabove-mentioned peak wavelengths.

<Condition 2-4>0.40≤[the total intensity of the L ₂ in the wavelength band ranging from−α_(R) to +α_(R) in the range of 600 nm to 780 nm/the total intensity ofthe L ₁ in the wavelength band ranging from −α_(R) to +α_(R) in therange of 600 nm to 780 nm]

Regarding the condition 2-4, it is more preferable to satisfy 0.45≤theright-hand side, and it is still more preferable to satisfy 0.47≤theright-hand side.

Further, in the condition 2-4, the definition as “the wavelength bandranging from −α_(R) to +α_(R) in the range of 600 nm to 780 nm” meansthat the wavelength band of less than 600 nm or more than 780 nm is notused for obtaining the total intensity, even if this wavelength band isin a range from −α_(R) to +α_(R).

The conditions 2-1 to 2-4 are the conditions for the wavelength band ofthe red (R) of L₂. For this reason, it is preferable to satisfy theconditions similar to the conditions 2-1 to 2-4, also in the wavelengthbands of the green (G) and blue (B) of L₂. Further, as described above,the shorter the wavelength becomes, the shorter the cycle of the opticalspectrum becomes, due to the influence of the retardation value, and thewavelength dependency of the birefringence index of the optical film,etc. Therefore, usually, when the conditions 2-1 to 2-4 are satisfied,the similar conditions are also satisfied in the wavelength bands of thegreen (G) and blue (B).

(Preferable Mode of L₁)

As described above, in the display device of the present invention,though the problem of the color reproducibility tends to occur with ausual design, since the condition 1-1 (the optical spectrum of L₁ issharp) is satisfied, the problem of the color reproducibility issuppressed by satisfying the condition 2-1.

Additionally, in the display device of the present invention, even ifthe optical spectrum of L₁ is very sharp, when the condition 2-1 issatisfied, it is possible to suppress the problem of the colorreproducibility. Recently, in order to extend the color gamut, displaydevices have been developed in which the optical spectrum of L₁ becomesvery sharp. The display device of the present invention is preferable inthat it is possible to suppress the problem of the colorreproducibility, even in display devices having a very sharp opticalspectrum of L₁.

For example, the display device of the present invention is preferablein that it is possible to suppress the problem of the colorreproducibility for the display device satisfying one or more of thefollowing condition 1-2 to the condition 1-5 (the display device havinga very sharp optical spectrum of L₁, and having a very wide colorgamut). The conditions 1-1 to 1-4 mainly contribute to expansion of thecolor gamut by increasing the color purity, and the condition 1-5 mainlycontributes to the expansion of the color gamut in consideration ofbrightness.

Further, it becomes easy to suppress the rainbow unevenness bysatisfying the condition 1-2.

<Condition 1-2>

Based on the optical spectrum of L₁ obtained by the measurement of thecondition 1-1, an average value B_(Ave) of the intensities of theoptical spectra in the blue wavelength band, an average value G_(Ave) ofthe intensities of the optical spectra in the green wavelength band, andan average value R_(Ave) of the intensities of the optical spectra inthe red wavelength band are calculated. Let B_(p) represent thewavelength band in which the intensities of L₁ continuously exceedB_(Ave) in the blue wavelength band, G_(p) represent the wavelength bandin which the intensities of L₁ continuously exceed G_(Ave) in the greenwavelength band, and R_(p) represent the wavelength band in which theintensities of L₁ continuously exceed R_(Ave) in the red wavelengthband. The numbers of wavelength bands indicating B_(p), G_(p) and R_(p)are all 1.

In the optical spectra of FIGS. 2, 4 and 5, the numbers of wavelengthbands indicating B_(p), G_(p) and R_(p) are all 1, and the condition 1-2is satisfied. On the other hand, in the optical spectrum of FIG. 3,there are two wavelength bands for each of B_(p) and G_(p), and thecondition 1-2 is not satisfied.

<Condition 1-3>

The above-mentioned +α_(B), the above-mentioned −α_(G), theabove-mentioned +α_(G), and the above-mentioned −α_(R) satisfy thefollowing relationships (5) to (6).+α_(B)<−α_(G)  (5)+α_(G)<−α_(R)  (6)

The optical spectra of FIGS. 2, 3 and 5 satisfy the relationships (5)and (6), and satisfy the condition 1-3. On the other hand, the opticalspectrum of FIG. 4 does not satisfy the relationship (6), and does notsatisfy the condition 1-3.

<Condition 1-4>

Let +β_(B) represent the minimum wavelength showing a ⅓ or less of theintensity of the B_(max), and located in the plus direction side ofL₁λ_(B), −β_(G) represent the maximum wavelength showing a ⅓ or less ofthe intensity of the G_(max), and located in the minus direction side ofL₁λ_(G), +βG represent the minimum wavelength showing a ⅓ or less of theintensity of the G_(max), and located in the plus direction side ofL₁λ_(G), and −β_(R) represent the maximum wavelength showing a ⅓ or lessof the intensity of the R_(max), and located in the minus direction sideof L₁λ_(R).

The above-mentioned +β_(B), the above-mentioned −β_(G), theabove-mentioned +β_(G), and the above-mentioned −β_(R) satisfy thefollowing relationships (7) to (8).+β_(B)<−β_(G)  (7)+β_(G)<−β_(R)  (8)

The optical spectra of FIGS. 2, 3 and 5 each satisfy the relationships(7) and (8), and satisfy the condition 1-4. On the other hand, theoptical spectrum of FIG. 4 does not satisfy the relationship (8), anddoes not satisfy the condition 1-4.

<Condition 1-5>

Let L_(1max) represent the maximum intensity among the above-mentionedB_(max), the above-mentioned G_(max), and the above-mentioned R_(max).B_(max)/L_(1max), G_(max)/L_(1max), and R_(max)/L_(1max) are each 0.27or more.

In the optical spectra of FIGS. 2, 3 and 5, B_(max)/L_(1max),G_(max)/L_(1max), and R_(max)/L_(1max) are each 0.27 or more, and thecondition 1-5 is satisfied. On the other hand, in the optical spectrumof FIG. 4, R_(max)/L_(1max) is each less than 0.27, and the condition1-5 is not satisfied.

In the condition 1-5, it is more preferable that B_(max)/L_(1max),G_(max)/L_(1max), and R_(max)/L_(1max) are each 0.30 or more.

Further, while it is preferable that the optical spectrum of L₁ be sharpfrom the viewpoint of extending the color gamut, it is preferable thatthe optical spectrum of L₁ be not extremely sharp from the viewpoint ofeasily satisfying the condition 2-1. For this reason, the differencebetween the above-mentioned +β_(R) and the above-mentioned −β_(R)[+β_(R)−(−β_(R))] is preferably 15 to 90 nm, more preferably 30 to 85nm, and still more preferably 50 to 80 nm.

Additionally, for the balance between the viewpoint of extending thecolor gamut and the viewpoint of easily satisfying the condition 2-3,the difference between the above-mentioned +α_(R) and theabove-mentioned −α_(R) [+α_(R)−(−α_(R))] is preferably 10 to 70 nm, morepreferably 20 to 60 nm, and still more preferably 30 to 55 nm.

As display devices having a very sharp optical spectrum of L₁, there area three-color independent type organic EL display device, a liquidcrystal display device using a quantum dot for a backlight, etc.

(Display Element)

As display elements, there are a liquid crystal display element, anorganic EL display element, an inorganic EL display element, a plasmadisplay element, etc. Further, the liquid crystal display element may bean in-cell touch-panel liquid crystal display element provided with atouch-panel function in the element.

Among these display elements, in the three-color independent typeorganic EL display element, the optical spectrum of L₁ easily becomessharp, and the effect of the present invention is effectivelydemonstrated. However, in organic EL display elements, there is aproblem of the optical extraction efficiency. Thus, in order to improvethe optical extraction efficiency, three-color independent type organicEL elements are provided with a micro-cavity structure. In thethree-color independent type organic EL elements provided with thismicro-cavity structure, the optical spectrum of L₁ tends to becomesharper as the optical extraction efficiency is more improved, and theeffect of the present invention is effectively demonstrated.

Additionally, when the display element is a liquid crystal displayelement, and a quantum dot is used as a backlight, the optical spectrumof L₁ easily becomes sharp, and the effect of the present invention iseffectively demonstrated.

As for the display element, the cover rate of BT. 2020 based on theCIE-xy chromaticity diagram represented by the above-mentioned formulais preferably 60% or more, and more preferably 65% or more.

(Polarizer a)

The polarizer a is placed on the emitting surface of the displayelement, and is located closer to the display element than the opticalfilm X.

As polarizers a, there are, for example, a polyvinyl alcohol film dyedwith iodine etc. and oriented, a sheet-type polarizer such as apolyvinyl-formal film, a polyvinyl-acetal film, and an ethylene-vinylacetate copolymer based saponificated film, a wire grid type polarizercomposed of many metal wires arranged in parallel, a coating typepolarizer to which a lyotropic liquid crystal or a dichroic guest-hostmaterial is applied, a multilayer thin film type polarizer, etc.Further, these polarizers a may be reflection-type polarizers providedwith the function of reflecting the polarization component that is nottransmitted.

It is preferable to cover both sides of the polarizer a with atransparent protective plate, such as a plastic film and glass. It isalso possible to use the optical film X as the transparent protectiveplate.

The polarizer a is used, for example, in order to add an antireflectionproperty by combination with a ¼λ plate. Additionally, when the displayelement is a liquid crystal display element, a backside polarizer isprovided on the light entering surface side of the liquid crystaldisplay element, and is used for adding the function of a liquid crystalshutter by arranging the absorption axis of the polarizer a located onthe upper side of the liquid crystal display element to be orthogonal tothe absorption axis of the backside polarizer.

In principle, the polarized sunglasses absorb the S polarization. Thus,in principle, the direction of the absorption axis of the polarizer ofthe polarized sunglasses is also horizontal. For this reason, it ispreferable to provide the polarizer a such that the angle of thedirection of the absorption axis of the polarizer a is within the rangeless than ±10 degrees with respect to the horizontal direction of thedisplay device. It is more preferable that the angle be within the rangeless than ±5 degrees.

When there are two or more polarizers between the display element andthe optical film X, the polarizer located farthest from the displayelement is assumed to be the polarizer a.

(Optical Film X)

The optical film X is on a surface on the light emitting surface side ofthe display element, and is provided farther from the light emittingsurface than the polarizer a. Additionally, when the display deviceincludes a plurality of polarizers, the optical film X is providedfarther from the light emitting surface than the polarizer (thepolarizer a) located farthest from the light emitting surface.

When a plurality of optical films is provided on the display element, itis preferable to provide the optical film X farthest from the displayelement (on the viewer side).

The optical film X has a role of changing the light before it transmitsthrough the optical film X, and making the relationship between L₁ andL₂ satisfy the condition 2-1.

Let “I₀” represent the intensity of L₁ incident vertically on theoptical film X, “Re₅₅₀” represent the retardation value of a wavelengthof 550 nm of the optical film X, “N(λ)” represent [the birefringenceindex of the material forming the optical film X with respect to each ofwavelengths ranging from 400 to 780 nm/the birefringence index of thematerial forming the optical film X with respect to the wavelength of550 nm], and “θ” represent the angle formed between the absorption axis(the vibrating direction of linear polarization) of polarizer of thepolarizer a and the slow axis of the optical film X. In this case, theintensity I of the light (L₂) that emits vertically from the lightemitting surface side of the optical film X, and passes through thepolarizer b having the absorption axis parallel to the absorption axisof the polarizer a can be represented by the following formula (A).Further, it is assumed that L₁ is the linear polarization that passesthrough the polarizer a located closer to the display element than theoptical film X.I=I ₀ −I ₀·sin²(2θ)·sin²(π·N(λ)·Re ₅₅₀/λ)  (A)

The configuration of the optical film X can be determined based on theabove formula (A). Specifically, first, the optical spectrum of L₁before being transmitted through the optical film X is measured. Then,based on the measurement result of the optical spectrum of L₁ and theabove formula (A), the simulation is performed for the optical spectrumof L₂ in accordance with the retardation value of the optical film X.Then, the optical spectrum of L₁ is compared with the optical spectrumof L₂ obtained in the simulation, and the optical film having theretardation that satisfies the condition 2-1 is determined as theoptical film X. By determining the configuration of the optical film Xin this manner, it is possible to achieve a good color reproducibility,without increasing the retardation of the optical film X more thannecessary.

Further, the value of I shows the maximum value when the angle θ formedbetween the absorption axis (the vibrating direction of linearpolarization) of polarizer of the polarizer a and the slow axis of theoptical film X is 45 degrees. For this reason, it is preferable toperform the above simulation with the following formula (B) assuming θto be 45 degrees.I=I ₀ −I ₀·sin²(π·N(λ)·Re ₅₅₀/λ)  (B)

When a plurality of optical films is provided on the display element, asdescribed above, it is preferable to provide the optical film X farthestfrom the display element (the viewer side). In this case, the abovesimulation may be performed by the interaction between the optical filmX and the optical film located closer to the display element thanoptical film X. For example, in a case where the slow axis direction ofthe optical film X is arranged to be the same as the slow axis directionof the optical film located closer to the display element than theoptical film X, when both of the optical films are formed from the samematerial, the above simulation can be performed by calculating Re₅₅₀based on the total thickness of both of the optical films.

When the retardation value of the optical film X is increased, itbecomes easy to satisfy the condition 2-1. However, a mere increase ofthe retardation value may not satisfy the condition 2-1. Additionally,when the retardation value of the optical film X is too large, thethickness of the optical film X may become too large, or it may benecessary to use a special material having a bad handleability as thematerial of the optical film X. In addition, when the retardation valueis too small, when observed through the polarized sunglasses, theblackout or the rainbow unevenness may easily occur.

For this reason, it is preferable to use the optical film X thatsatisfies the condition 2-1 in the range from 3,000 nm to 100,000 nm forthe retardation value. The retardation value of the optical film X morepreferably ranges from 4,000 nm to 30,000 nm, still more preferablyranges from 5,000 nm to 20,000 nm, still further preferably ranges from6,000 nm to 15,000 nm, and especially preferably ranges from 7,000 nm to12,000 nm. Further, the retardation value herein is a retardation valueat the wavelength of 550 nm.

The retardation value of the optical film X is represented by thefollowing formula (C) by using a refractive index n_(x) in the slow axisdirection, which is the direction having the largest refractive index inthe surface of the optical film, a refractive index n_(y) in the fastaxis direction, which is the direction orthogonal to the above-mentionedslow-axis direction in the surface of an optically transparent film, anda thickness d of the optical film.Retardation Value (Re)=(n _(x) −n _(y))×d  (C)

The above retardation value can be measured by “KOBRA-WR” and“PAM-UHR100” that are product names and made by Oji ScientificInstruments Co., Ltd., for example.

Additionally, after the orientation axis direction (the direction of themain axis) of the optical film X is obtained using two or morepolarizers, the refractive index (n_(x), n_(y)) of the two axes (therefractive index of the oriented axis, and the axis orthogonal to theorientation axis) is obtained with an Abbe refractive-index meter(NAR-4T, made by ATAGO CO., LTD). Here, the axis showing a largerrefractive index is defined as the slow axis. The thickness d of theoptical film is measured with, for example, a micrometer (the brandname: Digimatic Micrometer, made by Mitutoyo Corporation), and the unitis converted to nm. The retardation may also be calculated from theproduct of the birefringence index (n_(x)−n_(y)) and the thickness d ofthe film (nm).

As for the optical film X, there is the optical film X mainly formedfrom an optically transparent base material, such as a plastic film.

As the optically transparent base materials, there are oriented plasticfilms, such as a polyester film, a polycarbonate film, a cycloolefinpolymer film, and an acrylic film. Among these, from the viewpoint ofbeing easy to increase the birefringence index, a oriented polyesterfilm and a oriented polycarbonate film are preferable. Additionally,among the optically transparent base materials, those exhibiting thepositive dispersion property (the property in which the shorter thewavelength becomes, the larger the birefringence index becomes) arepreferable. Especially, the polyester film that is oriented (theoriented polyester film) has a strong positive dispersion property, andhas the property in which the shorter the wavelength becomes, the largerthe birefringence index becomes (the longer the wavelength becomes, thesmaller the birefringence index becomes). Therefore, even if theretardation value is equivalent to those of the other plastic films, theoriented polyester film is preferable in that the above condition 2-1can be easily satisfied. In other words, it is preferable to use theoriented polyester film as the base material of the optical film X,since the above condition 2-1 can be easily satisfied without increasingthe thickness of the base material more than necessary.

As the polyester film, a polyethylene terephthalate film (PET film), apolyethylene naphthalate film (PEN film), etc. are preferred.

As for the orienting, there are longitudinal uniaxial orienting, tenterorienting, successive biaxial orienting, and simultaneous biaxialorienting, etc.

Additionally, among the optically transparent base materials, thoseexhibiting a positive birefringence property are preferable from theviewpoint of mechanical strength. The optically transparent basematerial exhibiting a positive birefringence property means an opticallytransparent base material in which a refractive index n₁ of theorientation axis direction (the direction of the main axis) of theoptically transparent base material, and a refractive index n₂ of thedirection orthogonal to the orientation axis direction satisfy therelationship of n₁>n₂. As the optically transparent base materialexhibiting a positive birefringence property, there are polyester films,such as a PET film and a PEN film, aramid films, etc.

From the viewpoints of handleability and reduction of film thickness,the thickness of the optically transparent base material preferablyranges from 5 to 300 μm, more preferably ranges from 10 to 200 μm, andstill more preferably ranges from 15 to 100 μm.

The optical film X may include a function layer on the opticallytransparent base material. As the functional layer, there are a hardcoat layer, an anti-glare layer, an antireflection layer, an antistaticlayer, an antifouling layer, etc.

(Other Optical Films)

The display device of the present invention may include other opticalfilms, such as a phase difference film, a hard coat film, and a gasbarrier film. Further, it is more preferable that the other opticalfilms be provided closer to the display element than the optical film X.

(Touch Panel)

The display device of the present invention may be a display device witha touch panel, in which the touch pane is provide between the displayelement and the optical film X. Although the positional relationshipbetween the polarizer a and the touch panel on the display element isnot particularly limited, it is preferable to arrange the polarizer (thepolarizer a) located farthest from the light emitting surface betweenthe touch panel and the optical film X.

As the touch panel, there are a resistance film type touch panel, acapacitance type touch panel, an in-cell touch panel, an electromagneticinduction type touch panel, an optical touch panel, an ultrasonic typetouch panel, etc.

(Backlight)

When the display device is a liquid crystal display device, a backlightis disposed behind the display element.

As the backlight, any of an edge light type backlight and a directlyunder type backlight can be used.

As the light source of the backlight, there are an LED, a CCFL, etc.However, in the backlight using a quantum dot as the light source, theoptical spectrum of L₁ easily becomes sharp, and the effect of thepresent invention is effectively demonstrated.

The backlight using a quantum dot as the light source includes at leasta primary light source that emits primary light, and a secondary lightsource composed of the quantum dot that absorbs the primary light andemits secondary light.

When the primary light source emits the primary light having awavelength corresponding to blue, it is preferable that the quantum dot,which is the secondary light source, include at least one of a firstquantum dot that absorbs the primary light and emits the secondary lighthaving a wavelength corresponding to red, and a second quantum dot thatabsorbs the primary light and emits the secondary light having awavelength corresponding to green, and it is more preferable that thequantum dot include both of the above-mentioned first quantum dot andthe above-mentioned second quantum dot.

A quantum dot is a nanometer-sized semiconductor particle, exhibitsspecific optical and electric properties by the quantum confinementeffect (the quantum size effect) with which electrons and excitons areconfined in a nanometer-sized small crystal, and is also called asemiconductor nanoparticle and a semiconductor nanocrystal.

The material of the quantum dot is not particularly limited as long asthe material is a nanometer-sized semiconductor particle, and producesthe quantum confinement effect (the quantum size effect).

Quantum dots may be contained in the optical film that constitutes thebacklight.

[Method of Selecting Optical Film of Display Device]

A method of selecting the optical film of the display device of thepresent invention is a method of selecting the optical film of thedisplay device including the polarizer a and the optical film on asurface on the light emitting surface side of the display element, andwhen the light incident on the optical film satisfies the abovecondition 1-1, the method selects the optical film that satisfies theabove condition 2-1.

The optical spectra of L₁ and L₂ can be measured by using aspectrophotometer. At the time of measurement, the photodetector of thespectrophotometer is placed to be perpendicular to the light emittingsurface of the display device, and the viewing angle is set to 1 degree.Additionally, it is preferable that the light to be measured be thelight passing through the center of the effective display area of thedisplay device. Further, it is preferable to calculate the opticalspectrum of L₂ based on a simulation as described later.

It is preferable that the optical film satisfying the conditions 2-1 to2-4 be selected by the following procedures (a) and (b).

(a) Based on the measurement result of the optical spectrum of L₁measured in the condition 1-1, and the above formula (A), the opticalspectrum of L₂ in accordance with the retardation value of the opticalfilm X is calculated by the simulation. Further, instead of the aboveformula (A), the above formula (B) may be used.

(b) The optical spectrum of L₁ is compared with the optical spectrum ofL₂ calculated by the simulation, and the optical film having theretardation that satisfies the condition 2-1 is selected as the opticalfilm X.

According to the method of selecting the optical film of the displaydevice of the present invention, it is possible to efficiently selectthe optical film that can suppress the deterioration of the colorreproducibility during observation through the polarized sunglasses, andto improve the working efficiency.

The method of selecting the optical film of the display device of thepresent invention is especially effective when the optical spectrum ofL₁ is very sharp. Specifically, when the optical spectrum of L₁satisfies the above-mentioned conditions 1-2 to 1-5, since the problemof the color reproducibility are more aggravated, the method ofselecting the optical film of the display device of the presentinvention is very useful.

Additionally, from the viewpoint of making the color reproducibilitybetter, it is preferable for the method of selecting the optical film ofthe display device of the present invention to have one or moreconditions that are chosen from the above-mentioned conditions 2-2 to2-4, and it is more preferable to have all of the above-mentionedconditions 2-2 to 2-4.

EXAMPLE

Next, the present invention is described in more detail with examples.However, the present invention is not limited in any way by theseexamples. Further, “part” and “%” are based on a mass basis, unlessotherwise specified.

1. Fabrication of Optical Film

An non-oriented film was fabricated by melting polyethyleneterephthalate at 290° C., causing the polyethylene terephthalate to passthrough a film forming die to be extruded in a sheet-like shape, andcooling the polyethylene terephthalate by pressing the polyethyleneterephthalate on a rotating quenching drum that was water-cooled. Thisnon-oriented film was preheated for 1 minute at 120° C. in a biaxialorient test apparatus (TOYO SEIKI Co., Ltd), and was thereaftersubjected to a fixed-end uniaxial orienting of 4.0 times at 120° C., tofabricate the optical film having a birefringence property in a surface.The refractive indices are n_(x)=1.701 and n_(y)=1.6015, and thedifference between them is Δn=0.0995 at a wavelength of 550 nm.

The film thickness of this optical film was adjusted, and optical filmsi to vii having the following retardation values (Re) were obtained.

optical film i: Re=3,000 nm

optical film i: Re=4,000 nm

optical film iii: Re=6,000 nm

optical film iv: Re=7,000 nm

optical film v: Re=8,000 nm

optical film vi: Re=11,000 nm

optical film vii: Re=15,000 nm

2. Measurement of Optical Spectrum of L₁

The intensity of light (L₁) incident vertically on the optical film fromthe display element side was measured every 1 nm by using aspectrophotometer while the viewing angle was set to 1 degree, when thefollowing display devices A to E outputted white light. In the displaydevices A to E, the angle formed between the absorption axis (thevibrating direction of linear polarization) of the polarizer a and theslow axis of the optical film X was set to 45 degrees. Additionally, themeasurement position was the center of the effective display area of thedisplay device. FIG. 2 shows the optical spectrum of L₁ of the displaydevice A, FIG. 3 shows the optical spectrum of L₁ of the display deviceB, FIG. 4 shows the optical spectrum of L₁ of the display device C, FIG.5 shows the optical spectrum of L₁ of the display device D, and FIG. 14shows the optical spectrum of L₁ of the display device E. In addition,the numerical values about the conditions 1-1 to 1-5 calculated based onthe measurement results are shown in Table 1. Additionally, those thatsatisfies the conditions 1-1 to 1-5 are indicated by “◯”, those thatdoes not satisfy them are indicated by “x”, and both of them are alsoshown in Table 1.

<Display Device A>

A commercially available display device including the polarizer a andthe optical film on the three-color independent type organic EL displayelement having a micro-cavity structure. The cover rate of BT. 2020based on the CIE-xy chromaticity diagram: 77%.

<Display Device B>

A commercially available display device in which the display element isa liquid crystal display element with a color filter, and the lightsource of the backlight is a cold cathode fluorescent tube (CCFL), andthat includes the polarizer a and the optical film on the displayelement.

<Display Device C>

A commercially available display device in which the display element isa liquid crystal display element with a color filter, and the lightsource of the backlight is a white LED, and that includes the polarizera and the optical film on the display element. The cover rate of BT.2020 based on the CIE-xy chromaticity diagram: 49%.

<Display Device D (Display Device 1 Using Quantum Dot)>

A commercially available display device in which the display element isa liquid crystal display element with a color filter, the primary lightsource of the backlight is a blue LED, and the secondary light source isa quantum dot, and that includes the polarizer a and the optical film onthe display element. The cover rate of BT. 2020 based on the CIE-xychromaticity diagram: 68%.

<Display Device E (Display Device 2 Using Quantum Dot)>

A commercially available display device in which the display element isa liquid crystal display element with a color filter, the primary lightsource of the backlight is a blue LED, and the secondary light source isa quantum dot, and that includes the polarizer a and the optical film onthe display element. The cover rate of BT. 2020 based on the CIE-xychromaticity diagram: 52%.

TABLE 1 Display Display Display Device Device Device Display Display A BC Device D Device E B_(max) 100.0 54.3 100.0 100.0 100.0 G_(max) 72.0100.0 39.5 46.8 83.3 R_(max) 42.0 50.9 19.7 31.6 46.2 L₁λ_(B) 455 nm 435nm 450 nm 449 nm 448 nm L₁λ_(G) 525 nm 545 nm 532 nm 536 nm 548 nmL₁λ_(R) 620 nm 610 nm 640 nm 632 nm 611 nm −α_(B) 448 nm 431 nm 436 nm441 nm 440 nm +α_(B) 467 nm 440 nm 460 nm 459 nm 459 nm −α_(G) 514 nm541 nm 502 nm 516 nm 531 nm +α_(G) 538 nm 549 nm 568 nm 556 nm 564 nm−α_(R) 590 nm 607 nm 429 nm 606 nm 593 nm +α_(R) 636 nm 615 nm 695 nm661 nm 635 nm −β_(B) 446 nm 429 nm 436 nm 438 nm 437 nm +β_(B) 471 nm472 nm 466 nm 464 nm 464 nm −β_(G) 508 nm 539 nm 490 nm 510 nm 526 nm+β_(G) 545 nm 550 nm 680 nm 563 nm 569 nm −β_(R) 575 nm 606 nm 427 nm598 nm 588 nm +β_(R) 645 nm 620 nm 712 nm 669 nm 643 nm Number of 1 2 11 1 Wavelength Bands Indicating B_(p) Number of 1 2 1 1 1 WavelengthBands Indicating G_(p) Number of 1 1 1 1 1 Wavelength Bands IndicatingR_(p) Condition 1-1 ◯ ◯ X ◯ ◯ Condition 1-2 ◯ X ◯ ◯ ◯ Condition 1-3 ◯ ◯X ◯ ◯ Condition 1-4 ◯ ◯ X ◯ ◯ Condition 1-5 ◯ ◯ X ◯ ◯ Cover Rate of BT.77% — 49% 68% 52% 20203. Fabrication of Display Devices A-i to A-vii, Display Devices B-i toB-vii, Display Devices C-i to C-vii, Display Devices D-i to D-vii, andDisplay Devices E-i to E-vii

Display devices A-i to A-vii, display devices B-i to B-vii, displaydevices C-i to C-vii, display devices D-i to D-vii, and display devicesE-i to E-vii were obtained by arranging optical films i to vii as theoptical film of the display devices A to E.

4. Simulation of L₂, and Measurement of Optical Spectrum of L₂

Based on the optical spectrum of L₁ measured by above 2 and the aboveformula (B), the intensity I of light (L₂) was calculated by simulation.The light (L₂) is light that emits vertically from the light emittingsurface side of the optical film of the display devices A-i to A-vii,the display devices B-i to B-vii, the display devices C-i to C-vii, thedisplay devices D-i to D-vii, and the display devices E-i to E-vii, andthat passes through the polarizer b having the absorption axis parallelto the absorption axis of the polarizer a. The numerical values aboutthe conditions 2-1 to 2-4 calculated based on the simulation results areshown in Tables 2 to 6. Additionally, those that satisfies theconditions 2-1 to 2-4 are indicated by “◯”, those that does not satisfythem are indicated by “x”, and both of them are also shown in Tables 2to 6.

Further, the similar results were obtained when the numerical valuesabout the conditions 2-1 to 2-4 were calculated based on actualmeasurement values.

5. Evaluation

As described below, the display devices A-i to A-vii, the displaydevices B-i to B-vii, the display devices C-i to C-vii, the displaydevices D-i to D-vii, and the display devices E-i to E-vii wereevaluated. The results are shown in Tables 2 to 6.

5-1. Blackout

The screen of the display device was made white or substantially white.The screen was observed from various angles via the polarizedsunglasses, and it was visually evaluated whether there is any part inwhich the screen becomes dark.

◯: There is no part in which the screen becomes dark.

x: There is a part in which the screen becomes dark.

5-2. Rainbow Unevenness

The screen of the display device was made white or substantially white.The screen was observed from various angles via the polarizedsunglasses, and it was visually evaluated whether a rainbow patternunevenness is visually recognized.

◯: A rainbow pattern is not visually recognized.

Δ: A rainbow pattern is slightly visually recognized.

x: A rainbow pattern is visually recognized.

5-3. Color Reproducibility

The screen of the display device was made in color. The screen wasobserved from the front through the polarized sunglasses (state 1), andthrough a glass plate dyed the same color as that of the polarizedsunglasses placed on the screen without the polarized sunglasses (state2), and the color reproducibility with polarized sunglasses was visuallyevaluated.

Given 2 points for the case where the difference of color between thestate 1 and the state 2 (the difference of color based on red) is notnoticeable, 1 point for the case where the difference of color betweenthe state 1 and the state 2 (the difference of color based on red) isslightly noticeable, and 0 points for the case where the difference ofcolor between the state 1 and the state 2 (the difference of color basedon red) is very noticeable, 20 people made evaluations, and the averagepoint was calculated.

⊙: The average point is 1.7 or more

◯: The average point ranges from 1.5 to less than 1.7

Δ: The average point ranges from 1.0 to less than 1.5

x: The average point is less than 1.0

5-4. Sense of Presence of Moving Image

The screen of the display device was observed without the polarizedsunglasses while the screen displays a color moving image, and the senseof presence of the moving image was visually evaluated.

◯: The sense of presence is felt strong.

Δ: The sense of presence is felt.

x: The sense of presence is unsatisfactory.

TABLE 2 Display Device A-i A-ii A-iii A-iv A-v A-vi A-vii Number ofBottom 0 0 0 1 1 2 2 Wavelengths of Condition 2-1 Number of Peak 0 1 1 11 1 2 Wavelengths of Condition 2-1 Condition 2-1 X X X ◯ ◯ ◯ ◯Right-hand Side of 0.44 0.71 0.62 0.48 0.47 0.45 0.51 Condition 2-2Condition 2-2 ◯ ◯ ◯ ◯ ◯ ◯ ◯ Number of Bottom 0 0 0 1 1 2 1 Wavelengthsof Condition 2-3 Number of Peak 0 1 1 1 1 1 2 Wavelengths of Condition2-3 Condition 2-3 X X X ◯ ◯ ◯ ◯ Right-hand Side of 0.38 0.77 0.66 0.460.49 0.45 0.54 Condition 2-4 Condition 2-4 X ◯ ◯ ◯ ◯ ◯ ◯ Blackout ◯ ◯ ◯◯ ◯ ◯ ◯ Rainbow X Δ ◯ ◯ ◯ ◯ ◯ Unevenness Color X X X ◯ ◯ ◯ ⊙Reproducibility Sense of Presence ◯ ◯ ◯ ◯ ◯ ◯ ◯ of Moving Image

TABLE 3 Display Device B-i B-ii B-iii B-iv B-v B-vi B-vii Number ofBottom 0 0 0 0 1 1 1 Wavelengths of Condition 2-1 Number of Peak 0 1 0 00 1 1 Wavelengths of Condition 2-1 Condition 2-1 X X X X X ◯ ◯Right-hand Side of 0.23 0.96 0.66 0.65 0.14 0.45 0.51 Condition 2-2Condition 2-2 X ◯ ◯ ◯ X ◯ ◯ Number of Bottom 0 0 0 0 1 0 0 Wavelengthsof Condition 2-3 Number of Peak 0 1 0 0 0 0 1 Wavelengths of Condition2-3 Condition 2-3 X X X X X X X Right-hand Side of 0.19 0.99 0.59 0.750.05 0.32 0.61 Condition 2-4 Condition 2-4 X ◯ ◯ ◯ X X ◯ Blackout ◯ ◯ ◯◯ ◯ ◯ ◯ Rainbow Unevenness X X X X X X X Color Reproducibility X X X X XΔ Δ Sense of Presence of Δ Δ Δ Δ Δ Δ Δ Moving Image

TABLE 4 Display Device C-i C-ii C-iii C-iv C-v C-vi C-vii Number of 1 12 3 2 3 5 Bottom Wavelengths of Condition 2-1 Number of Peak 1 1 2 3 2 35 Wavelengths of Condition 2-1 Condition 2-1 ◯ ◯ ◯ ◯ ◯ ◯ ◯ Right-handSide 0.57 0.55 0.52 0.54 0.45 0.47 0.51 of Condition 2-2 Condition 2-2 ◯◯ ◯ ◯ ◯ ◯ ◯ Number of 1 1 2 2 2 3 4 Bottom Wavelengths of Condition 2-3Number of Peak 1 1 2 2 2 3 4 Wavelengths of Condition 2-3 Condition 2-3◯ ◯ ◯ ◯ ◯ ◯ ◯ Right-hand Side 0.61 0.52 0.54 0.50 0.47 0.49 0.50 ofCondition 2-4 Condition 2-4 ◯ ◯ ◯ ◯ ◯ ◯ ◯ Blackout ◯ ◯ ◯ ◯ ◯ ◯ ◯ Rainbow◯ ◯ ◯ ◯ ◯ ◯ ◯ Unevenness Color ◯ ◯ ⊙ ⊙ ⊙ ⊙ ⊙ Reproducibility Sense of XX X X X X X Presence of Moving Image

TABLE 5 Display Device D-i D-ii D-iii D-iv D-v D-vi D-vii Number ofBottom 0 1 1 1 2 3 3 Wavelengths of Condition 2-1 Number of Peak 1 1 1 21 2 3 Wavelengths of Condition 2-1 Condition 2-1 X ◯ ◯ ◯ ◯ ◯ ◯Right-hand Side of 0.69 0.42 0.48 0.53 0.47 0.48 0.50 Condition 2-2Condition 2-2 ◯ ◯ ◯ ◯ ◯ ◯ ◯ Number of Bottom 0 1 1 1 2 2 3 Wavelengthsof Condition 2-3 Number of Peak 1 1 1 1 1 2 3 Wavelengths of Condition2-3 Condition 2-3 X ◯ ◯ ◯ ◯ ◯ ◯ Right-hand Side of 0.72 0.40 0.49 0.520.45 0.52 0.51 Condition 2-4 Condition 2-4 ◯ ◯ ◯ ◯ ◯ ◯ ◯ Blackout ◯ ◯ ◯◯ ◯ ◯ ◯ Rainbow X Δ ◯ ◯ ◯ ◯ ◯ Unevenness Color X ◯ ◯ ◯ ◯ ⊙ ⊙Reproducibility Sense of Presence ◯ ◯ ◯ ◯ ◯ ◯ ◯ of Moving Image

TABLE 6 Display Device E-i E-ii E-iii E-iv E-v E-vi E-vii Number of 0 00 1 1 1 2 Bottom Wavelengths of Condition 2-1 Number of 0 0 1 1 1 1 2Peak Wavelengths of Condition 2-1 Condition 2-1 X X X ◯ ◯ ◯ ◯ Right-hand0.40 0.74 0.61 0.51 0.45 0.43 0.51 Side of Condition 2-2 Condition 2-2 ◯◯ ◯ ◯ ◯ ◯ ◯ Number of 0 0 0 1 1 2 1 Bottom Wavelengths of Condition 2-3Number of 0 0 1 1 1 1 2 Peak Wavelengths of Condition 2-3 Condition 2-3X X X ◯ ◯ ◯ ◯ Right-hand 0.35 0.80 0.64 0.49 0.45 0.44 0.55 Side ofCondition 2-4 Condition 2-4 X ◯ ◯ ◯ ◯ ◯ ◯ Blackout ◯ ◯ ◯ ◯ ◯ ◯ ◯ RainbowX Δ ◯ ◯ ◯ ◯ ◯ Unevenness Color X X X ◯ ◯ ◯ ⊙ Reproducibility Sense of ΔΔ Δ Δ Δ Δ Δ Presence of Moving Image

Referring to the results of Tables 1 to 6, in the display devicessatisfying the condition 1-1 and the condition 2-1 (the display devicesA-iv to A-vii, the display devices B-vi to B-vii, the display devicesD-ii to D-vii, and the display devices E-iv to E-vii), since the colorgamut is wide, excellent sense of presence for the moving image wasdemonstrated, and the problem of the color reproducibility, which tendsto occur when the color gamut is wide, could also be suppressed.

Additionally, among the display devices satisfying the condition 1-1 andthe condition 2-1, in the display devices further satisfying theconditions 1-2 to 1-5 and having the cover rate of BT. 2020 based on theCIE-xy chromaticity diagram of 60% or more (the display devices A-iv toA-vii, the display devices D-ii to D-vii), more excellent sense ofpresence for the moving image was demonstrated.

In addition, among the display devices satisfying the condition 1-1 andthe condition 2-1, in the display devices whose numbers of bottomwavelength and peak wavelength of the condition 2-3 are each one or more(the display devices A-iv to A-vii, D-ii to D-vii, E-iv to E-vii), moreexcellent color reproducibility was demonstrated. Among these, in thedisplay devices whose numbers of bottom wavelength and peak wavelengthof the condition 2-1 are each two or more (the display devices A-vii,D-vi, D-vii, E-vii), still more excellent color reproducibility wasdemonstrated.

REFERENCE SIGNS LIST

-   10: display element-   10 a: organic EL display element-   20: optical film X-   30: other optical films-   40: polarizer a-   100: display device

The invention claimed is:
 1. A display device comprising a displayelement, a polarizer A on a surface on a light emitting surface side ofthe display element and a plurality of optical films on the polarizer A,wherein when the optical film located farthest among the plurality ofoptical films on the polarizer A from the display element is defined asan optical film X, wherein the optical film X is a polyester film havingretardation value of 4,000 nm or more, wherein the display devicesatisfies the following Condition 1-1 and Condition 2-1: <Condition 1-1>let L₁ represent light incident vertically on the optical film X, amonglight incident on the optical film X from a display element side; anintensity of the L₁ is measured every 1 nm; a blue wavelength band rangeis from 400 nm to less than 500 nm, a green wavelength band range isfrom 500 nm to less than 600 nm, and a red wavelength band range is from600 nm to 780 nm; let B_(max) represent a maximum intensity of the bluewavelength band of the L₁, G_(max) represent a maximum intensity of thegreen wavelength band of the L₁, and R_(max) represent a maximumintensity of the red wavelength band of the L₁; let L₁λ_(B) represent awavelength showing the B_(max), L₁λ_(G) represent a wavelength showingthe G_(max), and L₁λ_(R) represent a wavelength showing the R_(max); let+α_(B) represent a minimum wavelength showing ½ or less of the intensityof the B_(max), and located in a plus direction side of L₁λ_(B), −α_(G)represent a maximum wavelength showing ½ or less of the intensity of theG_(max), and located in a minus direction side of L₁λ_(G), +α_(G)represent a minimum wavelength showing ½ or less of the intensity of theG_(max), and located in a plus direction side of L₁λ_(G), and −α_(R)represent a maximum wavelength showing ½ or less of the intensity of theR_(max), and located in a minus direction side of L₁λ_(R); and L₁λ_(B),L₁λ_(G), L₁λ_(R), +α_(B), −α_(G), +α_(G) and −α_(R) satisfy thefollowing relationships (1) to (4),+α_(B) <L ₁λ_(G)  (1)L ₁λ_(B)<−α_(G)  (2)+α_(G)<L ₁λ_(R)  (3)L ₁λ_(G)<−α_(R)  (4); <Condition 2-1> let L₂ represent light that emitsvertically from the light emitting surface side of the optical film X,and passes through a polarizer B having an absorption axis parallel toan absorption axis of the polarizer A; an intensity of the L₂ ismeasured every 1 nm; a wavelength at which an inclination of an opticalspectrum of the L₂ changes from negative to positive is a bottomwavelength, and a wavelength at which the inclination of the opticalspectrum of the L₂ switches from positive to negative is a peakwavelength; let −β_(R) represent a maximum wavelength showing ⅓ or lessof the intensity of the R_(max), and located in the minus direction sideof L₁λ_(R), and +β_(R) represent a minimum wavelength showing ⅓ or lessof the intensity of the R_(max), and located in a plus direction side ofL₁λ_(R); and a wavelength band ranging from −β_(R) to +β_(R) in therange of 600 nm to 780 nm includes one or more bottom wavelengths andone or more peak wavelengths, and wherein the display element is anorganic electroluminescent (EL) display element or an inorganicelectroluminescent (EL) display element.
 2. The display device accordingto claim 1, wherein the following Condition 1-2 is satisfied: <Condition1-2> based on an optical spectrum of L₁ obtained by the measurement ofthe condition 1-1, an average value B_(Ave) of intensities of an opticalspectrum in the blue wavelength band, an average value G_(Ave) ofintensities of an optical spectrum in the green wavelength band, and anaverage value R_(Ave) of intensities of an optical spectrum in the redwavelength band are calculated; let B_(p) represent a wavelength band inwhich the intensities of L₁ continuously exceed B_(Ave) in the bluewavelength band, G_(p) represent a wavelength band in which theintensities of L₁ continuously exceed G_(Ave) in the green wavelengthband, and R_(p) represent the wavelength band in which the intensitiesof L₁ continuously exceed R_(Ave) in the red wavelength band; and thenumbers of wavelength bands indicating B_(p), G_(p) and R_(p) are all 1.3. The display device according to claim 1, wherein the followingCondition 1-3 is satisfied: <Condition 1-3> the +α_(B), the −α_(G), the+α_(G) and the −α_(R) satisfy the following relationships (5) to (6),+α_(B)<−α_(G)  (5)+α_(G) <−α_(R)  (6).
 4. The display device according to claim 1, whereinthe following Condition 1-4 is satisfied: <Condition 1-4> let +β_(B)represent a minimum wavelength showing ⅓ or less of the intensity of theB_(max), and located in the plus direction side of L₁λ_(B), −β_(G)represent a maximum wavelength showing ⅓ or less of the intensity of theG_(max), and located in the minus direction side of L₁λ_(G), +β_(G)represent a minimum wavelength showing ⅓ or less of the intensity of theG_(max), and located in the plus direction side of L₁λ_(G), and −β_(R)represent a maximum wavelength showing ⅓ or less of the intensity of theR_(max), and located in the minus direction side of L₁λ_(R); and the+β_(B), the −β_(G), the +β_(G) and the −β_(R) satisfy the followingrelationships (7) to (8),+β_(B)<−β_(G)  (7)+β_(G)<−β_(R)  (8).
 5. The display device according to claim 1, whereinthe following Condition 1-5 is satisfied: <Condition 1-5> let L_(1max)represent the maximum intensity among the B_(max), the G_(max) andR_(max); and B_(max)/L_(1max), G_(max)/L_(1max), and R_(max)/L_(1max)are each 0.27 or more.
 6. The display device according to claim 1,wherein the following Condition 2-2 is satisfied: <Condition 2-2>0.40≤[a total intensity of the L ₂ in the wavelength band ranging from−β_(R) to +β_(R) in the range of 600 nm to 780 nm/a total intensity ofthe L ₁ in the wavelength band ranging from −β_(R) to +β_(R) in therange of 600 nm to 780 nm].
 7. The display device according to claim 1,wherein the following Condition 2-3 is satisfied: <Condition 2-3> let−α_(R) represent a maximum wavelength showing ½ or less of the intensityof the R_(max) , and located in the minus direction side of L₁λ_(R), and+α_(R) represent a minimum wavelength showing ½ or less of the intensityof the R_(max), and located in the plus direction side of L₁λ^(R); and awavelength band ranging from −α_(R) to +α_(R) in the range of 600 nm to780 nm includes one or more bottom wavelengths of the L₂ and one or morepeak wavelengths of the L₂.
 8. The display device according to claim 7,wherein a difference between the +α_(R) and the −α_(R) [+α_(R)−(−α_(R))]is 42 to 70 nm.
 9. The display device according to claim 1, wherein thefollowing Condition 2-4 is satisfied: <Condition 2-4>0.40≤[a total intensity of the L ₂ in the wavelength band ranging from−α_(R) to +α_(R) in the range of 600 nm to 780 nm/a total intensity ofthe L ₁ in the wavelength band ranging from −α_(R) to +α_(R) in therange of 600 nm to 780 nm].
 10. The display device according to claim 1,wherein the optical film X is an optically transparent base materialsubjected to uniaxial orienting.
 11. The display device according toclaim 1, further comprising a protective layer for the polarizer A otherthan optical film X.
 12. The display device according to claim 1,wherein a difference between the +β_(R) and the −β_(R) [+β_(R)−(−β_(R))]is 50 to 90 nm.
 13. A method of improving color reproducibility for adisplay device comprising a display element, and a polarizer A on asurface on a light emitting surface side of the display element, themethod comprising providing a plurality of optical films on thepolarizer A, wherein the plurality of optical films have properties suchthat, with the optical film located farthest among the plurality ofoptical films on the polarizer A from the display element designated asoptical film X, and light incident on the optical film X satisfying thefollowing Condition 1-1, the optical film X satisfies the followingCondition 2-1: <Condition 1-1> let L₁ represent light incidentvertically on the optical film X, among light incident on the opticalfilm X from a display element side; an intensity of the L₁ is measuredevery 1 nm; a blue wavelength band range is from 400 nm to less than 500nm, a green wavelength band range is from 500 nm to less than 600 nm,and a red wavelength band range is from 600 nm to 780 nm; let B_(max)represent a maximum intensity of the blue wavelength band of the L₁,G_(max) represent a maximum intensity of the green wavelength band ofthe L₁, and R_(max) represent a maximum intensity of the red wavelengthband of the L₁; let L₁λ_(B) represent a wavelength showing the B_(max),L₁λ_(G) represent a wavelength showing the G_(max), and L₁λ_(R)represent a wavelength showing the R_(max); let +α_(B) represent aminimum wavelength showing ½ or less of the intensity of the B_(max),and located in a plus direction side of L₁λ_(B), −α_(G) represent amaximum wavelength showing ½ or less of the intensity of the G_(max),and located in a minus direction side of L₁λ_(G), +α_(G) represent aminimum wavelength showing ½ or less of the intensity of the G_(max),and located in a plus direction side of L₁λ_(G), and −α_(R) represent amaximum wavelength showing ½ or less of the intensity of the R_(max),and located in a minus direction side of L₁λ_(R); and L₁λ_(B), L₁λ_(G),L₁λ_(R), +α_(B), −α_(G), +α_(G) and −α_(R) satisfy the followingrelationships (1) to (4),+α_(B) <L ₁λ_(G)  (1)L ₁λ_(B)<−α_(G)  (2)+α_(G) <L ₁λ_(R)  (3)L ₁λ_(G)<−α_(R)  (4); <Condition 2-1> let L₂ represent light that emitsvertically from the light emitting surface side of the optical film X,and passes through a polarizer B having an absorption axis parallel toan absorption axis of the polarizer A; an intensity of the L₂ ismeasured every 1 nm; a wavelength at which an inclination of an opticalspectrum of the L₂ changes from negative to positive is a bottomwavelength, and a wavelength at which the inclination of the opticalspectrum of the L₂ switches from positive to negative is a peakwavelength; let −β_(R) represent a maximum wavelength showing ⅓ or lessof the intensity of the R_(max), and located in the minus direction sideof L₁λ_(R), and +β_(R) represent a minimum wavelength showing ⅓ or lessof the intensity of the R_(max), and located in a plus direction side ofL₁λ_(R); and a wavelength band ranging from −β_(R) to +β_(R) in therange of 600 nm to 780 nm includes one or more bottom wavelengths andone or more peak wavelengths, and wherein the display element is anorganic electroluminescent (EL) display element or an inorganicelectroluminescent (EL) display element.